Electroless plating baths are used to coat non-metallic parts or parts having small intricate shapes. Once plating is initiated, the process is thermodynamically and kinetically driven and deposits a layer of metal uniformly over the immersed parts. Electroless plating is frequently employed to fabricate copper printed circuits.
Plating with an electroless bath, such as in the formation of printed circuits, requires careful sensitization of the substrates and vigilant maintenance of the bath constituents within narrow ranges as to proportionalities and temperature. Sensitization of the areas to be plated includes treatment with a solution of a colloidal metal and then an acceleration step to remove inactive components of the colloid from the sensitized substrate. After suitable rinsing, the sensitized substrate is immersed in the electroless bath where plating is initiated autocatalytically and builds to the desired thickness. Thereupon the substrate is removed and rinsed.
An effective and efficient plating process requires frequent monitoring to insure that the proportions of the constituents are maintained within a narrow tolerance. Although there are instruments and techniques to determine the approximate status of the baths, there occur undetected imbalances or variations in both the sensitization process or electroless bath composition that prevent the deposition of a uniform layer of metal over the entire seeded areas. Voids are typically the result.
One difficulty has been to accurately determine whether the electroless bath is in "take," that is, whether the bath will initiate and continue metal deposition on the seeded surface in a desired and normal manner. This has been done typically by removing and inspecting the immersed parts for coating quality but with the disadvantages that much time has been lost and, if the plating is defective or unsatisfactory, the work pieces must be scraped. Another method has been to measure resistance of the plating by determining the decrease in resistance of a plurality of seeded hole walls arranged in a series circuit, as in U.S. Pat. No. 4,477,484. This method shortens the determination time but still requires prolonged immersion to effect a relatively thick, low resistance coating. Some other monitoring schemes for autocatalytic plating baths are described in U.S. Pat. No. 3,375,178 where plating is noted to occur as long as bath pH is maintained and the difference in potential between a metal part to be plated and an inert reference electrode remains above a predetermined threshold; U.S. Pat. No. 4,331,699 where charge dissipation is observed with respect to time and reaction resistance is calculated a plurality of times to project a rate of plating; U.S. Pat. No. 4,182,638 where a metal substrate reacts with a constituent of a coating solution and the magnitude of current flow is used to maintain constituent concentrations; and U.S. Pat. No. 4,125,642 where potential difference between a conductive tank and non-catalytic probe is measured as an indication of accumulated decomposition products, but not plating activity. Such schemes, however, fail to suggest a manner in which to quickly determine the efficacy of an electroless plating bath with respect to seeded non-metallic substrates.